In this application we have outlined a series of studies directed at establishing new concepts on the central organization of visually evoked movements. Through the experiments described in Part I, we hope to identify one of the processes underlying arm movements in monkeys. Specifically, we want to find which parameters of movement are specified by the central commands. To this end we will observe the effects of a change in the initial position of the forearm on the attainment of an "intended" final arm position in both intact and deafferented monkeys. Because the velocity of the moving arm might be programmed separately, we will attempt to define the presumably distinct characteristics of the process underlying arm velocity. In addition, we will strive to understand the nature of the interactions between the central processes underlying arm movements and afferent feedback. To this end, we will evaluate the effectiveness of the mechanism by which externally applied loads are offset and more importantly, the role played by the feedback system in updating and adjusting the execution of learned motor patterns when posture in changed. In the second part of this proposal, we have outlined a series of experiments that are aimed at characterizing head-related cells in the brain stem, both within and outside the newly discovered head-related area in the pons. We plan to investigate in both intact and deafferented monkeys, the afferent input to these cells and their patterns of activity during eye and head movements. Initially, we will concentrate on establishing whether these cells can be classified according to the following distinct categories: (1) neurons that relay supramesencephalic commands, (2) cells that belong to internal feedback loops and (3) cells that are part of local spino-reticular spinal circuits. In accordance with the concepts outlined in Part I, we will investigate the way in which the supraspinal mechanism elaborates descending signals that determine the shift in posture. We will also observe the variations in brain stem activity associated with changes in head velocity and compensation for externally applied loads.